The present invention relates to the field of fiber optic testing equipment and, in particular, to an optical fault locator for locating faults in fiber optic cables.
Over the past ten years, fiber optic cables have replaced traditional telephone cables as the cable of choice for telecommunication. Although fiber optic cables have many advantages over traditional copper cables, fiber optic cables are still subject to breakage or other damage during installation or use. Accordingly, the demand for test equipment capable of detecting and locating faults in fiber optic cables has increased in recent years.
Currently, the two primary types of equipment for detecting and locating faults in fiber optic cables are optical time domain reflectometers (OTDR""s) and optical fault locators. An OTDR is an optoelectronic instrument that characterizes an optical fiber by injecting a series of optical pulses into the fiber under test, extracting light that is backscattered and reflected back, measuring and integrating the intensity of the return pulses as a function of time, and plotting the integration as a function of fiber length. From this plot, the fiber""s length, overall attenuation, including splice and mated-connector losses, and the location of any faults or breaks may be estimated.
Backscattered light, commonly referred to as Rayleigh scattering, is typically weak, and is due to refractive index fluctuations and inhomogeneities in the fiber core. The strength of the backscattered signal is primarily dependent upon the peak power and width of the test pulse. The backscattered signal may be used to detect faults such as micro-bends or splice losses, and to measure overall attenuation.
Reflective signals, commonly referred to as Fresnel reflections, are somewhat stronger and are caused by discontinuities in the fiber. The strength of the reflected signal is primarily dependent upon the peak power of the test pulse. Reflective signals may be used to determine the overall length of the fiber line, and to detect breaks in the fiber reflective connectors and splices of fibers having different indices of refraction.
As noted above, typical OTDR""s analyze both backscattered and reflected signals and plot this analysis on a display, similar to an oscilloscope, for interpretation by a user. Although such a display allows a user to determine a broad range of information relating to the fiber optic cable, users must be skilled in the use of the OTDR in order to determine the specific location of any given fault. In addition, the cost of an OTDR typically ranges from about $7,000 to $50,000, depending upon the desired features and accuracy. This relatively high cost may be justified in applications, such as network installation and optimization, where full functionality is required. However, a large number of maintenance operations require only the detection of a fault location, and not the calculation of other sources of attenuation.
The high cost of OTDR""s, their use as dedicated fault locators, and the high skill level required to accurately determining a fault location, has led to the development and manufacture of optical fault locators. Optical fault locators offer a lower cost alternative to OTDR""s by providing only the fault location feature of an OTDR. A typical optical fault locator measures the distance to an optical fault by sending a light pulse through the fiber optic cable, measuring the time that passes between sending the pulse and receiving the reflected return pulse, and calculating the distance to the reflection point using the equation d=(c/IOR)*(xcex94t/2); where d is the distance to the fault, c is the speed of light, IOR is the index of refraction, and xcex94t is the time period between sending and receiving of the pulse. In most optical fault locators, the resulting distance value is then presented on a liquid crystal display, in units of feet or meters, for use by the operator.
Although typical optical fault locators provide a sufficient degree of accuracy for many applications, there have had a number of drawbacks that have limited their popularity. First, typical optical fault locators have a single pre-programmed index of refraction, while different fiber optic cables will have different indices of refraction. Therefore, in order to determine the fault distance for a particular cable, the value given by a typical locator must be multiplied by an IOR factor that will correct the distance based upon the actual IOR of the cable being tested. Second, because different types of faults will cause different degrees of reflection, typical optical fault locators include a means for manually adjusting the sensitivity of the locator in order to find a known fault. Unfortunately, this manual adjustment requires a high degree of experience on the part of the user in order to find the desired fault and to avoid false fault readings. Third, some optical fault locators only provide results in either feet or meters, as the calculations required to provide a dual display are not easily performed by the circuitry commonly used in these locators. Although, some locators accommodate both feet and meters through use of dual oscillators, the use of these oscillators tends to increase the cost and weight and size of the unit. Fourth, most optical fault locators use reflection amplitude to differentiate between various fiber reflection events in the cable under test, requiring expensive analog laser power controls and expensive laser light detector threshold setting controls to provide suitable differentiation between reflection events. Finally, current optical fault locators are operated at high frequency and produce significant levels of electromagnetic interference (EMI). This EMI must be shielded to avoid errors within the locators and to meet FCC interference requirements.
Therefore, there is a need for an optical fault locator that has a significantly lower cost than typical OTDR""s, that calculates distance to a fault within an acceptable degree of accuracy, that allows the index of refraction to be varied by the user, that does not require user adjustment of the sensitivity of the unit in order to find a fault, that provides a display in feet or meters without the use of separate oscillators, does not require the use of expensive analog laser controls to differentiate between reflection events, and does not produce significant amounts of EMI.
The present invention is a hand held optical fault locator used to determine the distance to a discontinuity in a fiber optic cable. A discontinuity may be a fault, such as a break, nick, cut, scrape, indentation, or the like, a splice, a connector, or the end of the cable. The preferred optical fault locator includes an analog front end, a complex programmable logic device (CPLD), a laser diode and drive, a microcontroller, at least one input, and at least one output.
In operation, the user will input information through the input into the microcontroller, which sends a signal to the laser drive and diode to launch a pulse of laser light into the fiber optic cable. Once the pulse of laser light reaches a fault, such as a break, nick, cut, scrape, indentation, splice, connector, or the end of the cable, a portion of the light is reflected back through the cable and into the analog front end. The analog front end receives the reflected light generates an electrical signal corresponding to the intensity of the reflected light, amplifies this signal, and compares the amplified signal to a threshold value provided by the microcontroller. If the signal exceeds the threshold value, a digital pulse is sent to the microcontroller through the CPLD informing the microcontroller of time period between firing and reception. The time period is recorded and stored for later use by the microcontroller to determine the distance to the fault. The optical fault locator repeats this process for a predetermined number of pulses with the time periods between each being stored by the microcontroller. Once all of the pulses have been launched and light received, the microcontroller integrates them into the overall length count value, in accordance with a preprogrammed algorithm, to calculate the distance to the fault.
The preferred optical fault locator has a number of features that overcome drawbacks of prior art OTDR""s and optical fault locators. First, the preferred optical fault locator has a user adjustable IOR feature that allows the IOR to be selected by the operator. This feature makes use of a programmable oscillator to adjust for different user-input IOR values. Once inputted by the user, a microcontroller references a look-up table corresponding to the selected units of measure and matches a frequency of oscillation corresponding to the selected IOR and selected units. The programmable oscillator then sends a counting signal in which one count is equal to one of the desired units of measure; i.e. one foot or one meter.
Second, the use of a programmable oscillator allows measurements to be taken directly in either feet or meters, eliminating the need for dual oscillators or complex divide chain circuits to provide the desired output in feet or meters.
Third, the preferred optical fault locator determines the time to a reflective event using a low-frequency integrating technique in which the base clock is {fraction (1/25)} slower than a 1:1 clocking scheme. This permits high accuracy fiber lengths to be measured at counting frequencies in the 4 MHz range, eliminating the need for high frequency, EMI producing, circuitry.
Fourth, the preferred optical fault locator eliminates the need for expensive laser power control circuitry by determining fiber reflection events in the digital-time domain. Rather than continuously pinging, as is the case with common OTDR""s and optical fault locators, the optical fault locator of the present invention sends a series of bursts and stops. This conserves power, allowing the laser to be launched at full power without the undesirable battery drain caused by continuous pinging.
Fifth, the preferred optical fault locator operates a trans-impedance amplifier at maximum saturating gain, allowing fiber reflection events to be determined by counting them digitally. This eliminates the need for a sensitivity adjustment feature and for expensive analog variable gain control circuitry.
Finally, in some alternative embodiments of the invention where the trans-impedance amplifier is not operated at maximum saturation gain, a threshold control algorithm is utilized to eliminate the need for a sensitivity adjustment feature.
Therefore, it is an aspect of the invention to provide an optical fault locator that has a significantly lower cost than typical OTDR""s.
It is a further aspect of the invention to provide an optical fault locator that calculates distance to a fault within an acceptable degree of accuracy.
It is a further aspect of the invention to provide an optical fault locator that allows the index of refraction to be varied by the user.
It is a further aspect of the invention to provide an optical fault locator that does not require manual adjustment of the sensitivity of the unit in order to find a fault.
It is a further aspect of the invention to provide an optical fault locator that utilizes an averaging technique to eliminate noise.
It is a further aspect of the invention to provide an optical fault locator that provides a display in feet or meters without the use of separate oscillators.
These aspects of the invention are not meant to be exclusive and other features aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims and accompanying drawings.